Field of the Invention
The present invention relates to a refractive index distribution measuring method and a refractive index distribution measuring apparatus for measuring a refractive index distribution of an optical element.
Description of the Related Art
Mold-based lens manufacturing methods offer an advantage of expedited mass production of optical lenses, but may cause a refractive index distribution within a lens. The refractive index distribution caused within a lens adversely affects an optical performance of the lens. Therefore, the method of manufacturing lenses by mold requires a technique to non-destructively measure a refractive index distribution within a lens manufactured by molding.
A measuring method discussed in U.S. Pat. No. 5,151,752 includes immersing a test object and a glass sample, the refractive index and the shape of which are known, in a first matching fluid having a refractive index approximately equal to the refractive index of the test object, allowing light to pass through them, and thus measuring interference fringes. The measuring method further includes immersing the test object and the glass sample in a second matching fluid having a refractive index slightly different from the refractive index of the first matching fluid, allowing light to pass through them and thus measuring interference fringes. Then, according to this measuring method, the shape and the refractive index distribution of the test object are obtained based on the result of measurement using the first matching fluid and the result of measurement using the second matching fluid. The refractive index of each of the first and second matching fluids is required to slightly differ from the refractive index of the test object to the extent that the interference fringes do not become too dense.
A measuring method discussed in U.S. Pat. No. 8,472,014 includes arranging a test object in a medium having a refractive index different from the refractive index of the test object, and measuring a first transmitted wavefront for a first wavelength and a second transmitted wavefront for a second wavelength different from the first wavelength. Then, according to this measuring method, the refractive index distribution of the test object is calculated by removing a shape component of the test object using results of measurement of the first transmitted wavefront and the second transmitted wavefront and respective transmitted wavefronts for the first wavelength and the second wavelength of a reference test object arranged in the medium, the reference test object having the same shape as the test object and a specific refractive index distribution.
The measuring method discussed in U.S. Pat. No. 5,151,752 requires a matching fluid having a refractive index approximately equal to the refractive index of the test object. However, a matching fluid having a high refractive index is low in transmittance. Therefore, the measuring method discussed in U.S. Pat. No. 5,151,752, when measuring interference fringes occurring in an optical element having a high refractive index, allows only a small signal to be output from a detector, and thus becomes low in measurement accuracy.
The measuring method discussed in U.S. Pat. No. 8,472,014 is based on the premise that the refractive index (phase refractive index) of the reference test object is known. The phase refractive index of the reference test object needs to coincide with the phase refractive index of a point (for example, the center of a lens) within the test object. Therefore, the refractive index distribution measuring method discussed in U.S. Pat. No. 8,472,014 requires a technique to non-destructively measure the phase refractive index of a point within the test object. However, it is difficult to measure the phase refractive index in a non-destructive manner. A low coherence interference method and a wavelength scanning interference method are may be used to measure the refractive index in a non-destructive manner, but the measured refractive index is not a phase refractive index but a group refractive index. The phase refractive index and the group refractive index are not in one-to-one correspondence with each other, so that a phase refractive index obtained by converting a group refractive index contains a conversion error.
The phase refractive index Np(λ) is a refractive index related to the phase velocity vp(λ), which is the moving velocity of the equiphase surface of light. The group refractive index Ng(λ) is a refractive index related to the moving velocity vg(λ) of energy of light (the moving velocity of a wave packet).